When a plurality of normally independent power modules are connected in a parallel configuration, one of the power modules typically supplies almost all of the load current, leaving the rest of the modules poorly utilized, resulting in uneven distribution of electrical and thermal stress, and consequent decrease in the life of the entire power supply. For example, consider a case wherein two power modules are connected together in parallel to drive a single load. A first module with a higher output voltage than a second module will supply all the current to the load so long as the demand for current does not exceed the maximum current capacity of the first module. The second module will provide current only when the load overwhelms the capacity of the first module, causing the output voltage of the first module to drop. The result is severely unbalanced sharing of the load current between the two modules.
In a power supply with multiple paralleled power modules, it is known to include circuitry for equally distributing a load current among a plurality of power modules. This technique is referred to as "load sharing". Load sharing is typically found in power supplies wherein more than one power module services a single load or a common point on a power bus which then supplies multiple loads.
A known method for current sharing is referred to as the "master-slave" approach wherein a preselected master supply controls all the other supplies, called slaves. Each slave supply delivers an equal share of the total load current. However, if the master fails, the entire system becomes disabled, defeating the purpose of a redundant supply.
Another method for current sharing employs an external load sharing module. A cable from each power module is connected to the load sharing module, and a single pair of cables connect the load sharing module to a supply bus. Control circuitry within the load sharing module feeds back to the power modules to insure load balancing. The load sharing module is built to accommodate a fixed number of power modules. If additional modules need to be added to the system, the load sharing module must be modified or replaced.
Yet another method for implementing current sharing is referred to as "current averaging", as exemplified in U.S. Pat. No. 4,717,833. With this approach, each power supply module drives a common share bus via a resistor. If each supply is sourcing the same amount of load current, then each will drive the common bus with the same voltage, resulting in negligible current flow through each resistor driving the common bus. A differential measurement of the bus resistance determines the extent to which a reference signal is adjusted. A problem arises with this method when relatively low capacity supplies are used in conjunction with relatively higher capacity supplies. This problem occurs when there is one module that, although operational, cannot source its required share of the load current. In this case, the bus is dragged down, and the reference adjust circuitry within each of the other modules on the bus clamps to the low value. This results in an undesirable change in output voltage along with unequal current distribution among the supplies.